CD19 is a surface protein found on B cells and on certain cancerous cells derived from B cells, such as many B cell lymphomas. Anti-CD19 monoclonal antibodies have been generated in mice, and show some promise in pre-clinical animal models of B cell-derived cancers. However, mouse-derived antibodies are generally immunogenic in humans. A number of strategies have been developed to alter mouse-derived antibodies to minimize their immunogenicity in humans. One such strategy, chimerization, involves the fusion of mouse variable regions to human constant regions. However, the mouse-derived variable region sequences remaining following chimerization will often be immunogenic. Another such strategy, humanization, involves the replacement of mouse-derived framework regions (FRs) within the variable regions with the most closely related human-derived sequences, with the optional reversion of certain amino acids back to the corresponding mouse amino acid in order to maintain binding activity. However, even humanized antibodies may be immunogenic, since the antibody complementarity determining regions (CDRs) generally contain B cell epitopes and T cell epitopes that are non-self. Indeed, even fully human antibodies are immunogenic; this is the basis for the formation of anti-idiotype antibodies during the course of an immune response. All of these problems may apply to mouse-derived anti-CD19 antibodies as they would to any other type of antibody. Therefore, there is a need for anti-CD19 antibodies with reduced immunogenicity.